1. Field of the Invention
The present invention relates to a silicon carbide semiconductor device.
2. Description of the Background Art
Schottky barrier diodes (SBDs) are unipolar devices and are therefore capable of reducing switching loss compared with that of ordinary bipolar diodes. However, the conventional SBDs formed of silicon (Si) semiconductors only provide a breakdown voltage of about 50 V or less for practical use, thereby being unsuitable for uses such as high-voltage inverters. Therefore, attention has been given to the development of SBDs formed of silicon carbide (SiC-SBDs) in recent years because the use of silicon carbide (SiC), in place of silicon, as the constituent material of the SBDs allows for a breakdown voltage of about several kV.
As the technique known for improving the breakdown voltage in the SiC-SBDs, the P-type guard ring region (terminal well region) is provided in the so-called terminal region of the N-type silicon carbide semiconductor layer, so that the electric filed generated in application of a reverse voltage is alleviated by the depletion layer formed by the PN junction between the silicon carbide semiconductor layer and the guard ring region (for example, Japanese Patent Application Laid-Open No. 2005-286197).
Meanwhile, an etching residue is, in some cases, formed at the peripheral edge of the Schottky electrode disposed on the surface of the silicon carbide semiconductor layer. The formation of etching residue may cause the silicon carbide semiconductor device to malfunction. Therefore, the front-surface electrode disposed on the Schottky electrode covers the peripheral edge of the Schottky electrode, so that the etching residue formed at the peripheral edge of the Schottky electrode is prevented from being exposed. This technique has been known for suppressing malfunction of silicon carbide semiconductor devices (see, for example, Japanese Patent Application Laid-Open No. 2013-211503).
The technique of providing the high-concentration terminal well region having the higher P-type impurity concentration in the terminal well region has been known for further improving the breakdown voltage (see, for example, Japanese Patent Application Laid-Open No. 2008-251772).
Even in such a silicon carbide semiconductor device, however, it is newly found that, in switching from the conduction state in which an on-state current flows to the blocking state in which a reverse voltage is applied, the electric field may be concentrated in the peripheral edge of the front-surface electrode, possibly causing breakdown voltage failure. The electric field concentration in the peripheral edge of the front-surface electrode in switching is assumed to occur by the mechanism described below.
When the conduction state changes to the blocking state in which a reverse voltage is applied, the voltage applied onto the silicon carbide semiconductor device increases and varies, thereby generating a displacement current that charges the depletion-layer capacitance formed in the PN junction portion between the terminal well region and the silicon carbide semiconductor layer. The displacement current flows from the inside of the terminal well region toward the Schottky-electrode side. The flow of the displacement current causes a voltage drop in the terminal well region because the terminal well region has the specific resistance value. This causes a difference in electric potential between the inside of the terminal well region and the Schottky electrode, whereby an electric field is generated and the electric field concentration occurs in the peripheral edge of the Schottky electrode.
The electric field generated in switching is determined by the magnitude of displacement current and the resistance value in the terminal well region. The SiC-SBD is a unipolar device and is therefore capable of switching at an increased speed compared to that of the silicon diode having the same breakdown voltage. Thus, the voltage variation in switching becomes larger, resulting in an increase in displacement current value. In addition, the silicon carbide semiconductor has a large difference in energy level between the acceptor and the valence band, so that the resistance value in the terminal well region of the silicon carbide semiconductor is larger than that of the conventional silicon semiconductor. Therefore, both the displacement current value and the resistance value in the terminal well region are large in the SiC-SBD, thereby particularly increasing the electric field generated in switching. Thus, the electric field concentration in switching has been likely to cause element failure in the conventional SiC-SBD.